<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    手机号码：<input type="number"><button>发送</button>
    <script>
        var btn = document.querySelector('button');
        var time = 3;
        btn.addEventListener('click', function() {
            btn.disabled = true;
            var timer = setInterval(function() {
                if (time == 0) {
                    clearInterval(timer);
                    btn.disabled = false;
                    btn.innerHTML = '发送';
                    time = 3;
                } else {
                    btn.innerHTML = '还剩下' + time + '秒';
                    time--;
                }

            }, 1000);

        })
    </script>
</body>

</html>